Nucleotide analogues which are synthesized by the novel method of this invention are known per se. They are described for example in U.S. Pat. Nos. 4,659,825 and 5,142,051, WO 92/08686 and EP 269,947, 454,427 and 452,935. Various synthetic routes are known. See for instance U.S. Pat. No. 5,130,427; EP 270,885 and WO 92/02511. These nucleotide analogues are known to possess antiviral activity, e.g., as described in U.S. Pat. No. 4,724,233.
Nucleotide analogues of particular interest are purin-9-yl and pyrimidine-1-yl substituted phosphonylmethoxyalkyl compounds (U.S. Pat. Nos. 5,142,051 and 4,808,716), in particular 9-(phosphonylmethoxyethyl)-substituted bases, especially 9-substituted adenine ("PMEA" and "PME"-type analogues), (3-hydroxy-2-phosphonylmethoxy)propyl-substituted bases, especially 1-substituted cytosine ("HPMPC" and "HPMP"-type analogues and 2-phosphonylmethoxypropyl-substituted bases, "PMP"-type analogues), all of which contain an acyclic linkage between the base and the phosphonate moiety.
A known method for the preparation of PMEA involves the reaction of suitably activated alcohols or akyl halides with salts of adenine, usually in dimethylformamide ("DMF") to yield 9-hydroxyethyladenine. See A. Holy, "Collect. Czech. Chem. Commun." 43:3444 (1978); 44:593 (1979); 43:3103 (1978) and 43:2054 (1978). The amino group of 9-hydroxyethyladenine is blocked (U.S. Pat. No. 4,808,716, column 2, lines 58-66 and Holy et al., "Coll. Czech. Chem. Commun." 52:2801 et seq. 1987!). The protected, alkylated base then is reacted with an alkyl diester of p-toluenesulfonyloxymethanephosphonic acid in the presence of sodium hydride in a dipolar aprotic solvent such as DMF. The product is purified on ion exchange and gel chromatography columns.
Another method for the synthesis of nucleotide analogues of the PME type is described in Martin, ACS Symposium Ser. 401, pp.72-87 (1989). This method involves coupling of the heterocyclic base with a substituted alkyl which bears the phosphonylmethyl ether group. This procedure starts with ring opening of 1,3-dioxolane followed by 3 additional steps and then reaction of the intermediate phosphonate with unprotected adenine. This method, however, produces a mixture of 7- and 9-substituted adenyl compounds. Since these isomers are quite difficult to separate, expensive and exhaustive chromatography steps are required to produce the desired N.sup.9 derivative.
Thus, the known synthetic routes to the desired analogues suffer from various disadvantages, including costly starting reagents and the need to conduct complex synthetic procedures.
Accordingly, it is an object of this invention to provide a facile preparative method for the analogues of interest.
It is another object of this invention to dispense with the numerous steps characterizing prior methods and in particular to avoid the need to conduct burdensome and expensive chromatographic procedures.
It is a further object to improve the yields of analogue while minimizing impurities and side-products, e.g. regioisomers such as N.sup.7 -adenyl.
An additional object is to design a synthetic method which produces only crystalline intermediates and therefore simplifies the procedure.
Another object is to obviate the need to employ dioxolane as a starting material.
Another object is to avoid the need for protection and deprotection of amino groups of nucleoside bases.
A still further object is to improve the economics of the preparation of the analogues.